Background: Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the CFTR gene, that codes for a chloride channel located in the apical surface of epithelial cells. The main role of this protein is the regulation of chloride transport, and secondarily, of sodium and water to the extracellular space. More than 900 gene mutations have been described, and their relative frequency in different populations depends on their ethnic origin. Aim: To report the findings of Chilean patients with cystic fibrosis, in whom the presence of 20 common mutations was analyzed. Patients and methods: Fifty seven patients with established diagnosis or suspicion of CF were studied. The simultaneous identification of 20 mutations and the normal DF508 allele was done using polymerase chain reactions with a commercial assay. Results: Eight mutations were found. Fifty patients fulfilled diagnostic criteria proposed by the Consensus Panel of the CF Foundation and 66% of alleles were identified in this group. ∆F508 mutation was found in 45%. We did not identify mutations in any of the remaining 7 patients. Conclusions: Our results suggest that the majority of undetected mutations are associated with atypical phenotypes or that some patients in this series could have other diseases. We recommend to include mutation analysis in the evaluation of Chilean patients with CF. It is useful to establish prognosis and genetic counselling (Rev Méd Chile 2001; 129: 841-7).